Increasingly, modern communications systems transmit and receive information by modulating a radio frequency (RF) carrier signal with a data signal and by demodulating the RF carrier signal to recover the data signal, respectively. In order to demodulate the RF carrier signal, a receiver is tuned to the carrier signal.
One technique for modulating and demodulating such signals involves heterodyning, which involves mixing a received signal with a second signal to produce an intermediate signal at an intermediate frequency (IF). Typically, heterodyning systems multiply one signal, such as an RF signal, with a second signal close in frequency to the first, often referred to as a local oscillator (LO) signal. This operation produces signals at frequencies equal to the sum and difference of the RF frequency and the LO frequency. The sum frequency is usually substantially higher than the RF or LO frequency and may be readily filtered using a simple low-pass filter. The difference frequency is the IF frequency, which is usually close to DC and is therefore readily manipulated by simple filters.
Unfortunately, a desired RF frequency may lie either above or below the LO frequency. Due to the symmetric properties of the mixing operation, heterodyning systems may select any RF signal differing from the LO frequency by the IF frequency, whether the selected signal lies above or below the given LO frequency. For example, if a desired signal is at 1.01 GHz and the LO signal is at 1.00 GHz, mixing the two signals results in an IF frequency of 0.01 GHz. If two RF signals are present (one at 0.99 GHz and one at 1.01 GHz), they may both be converted to the 0.01 GHz IF frequency, thereby resulting in interference with the information content of the desired RF signal. In general, receivers that use heterodyning techniques are susceptible to “imaging.” The image frequency is equal to the sum or the difference of the LO frequency and the IF frequency (fimage=fLO±fIF).
One technique for addressing the phenomenon of imaging is described in U.S. Patent Publication Nos. 2005/0070239A1 and 2005/0070236A1, both of which were invented by Tod Paulus on Sep. 29, 2003, and are incorporated herein by reference.
In mobile communication devices, voltage supplies are tightly controlled in order to reduce power consumption and lengthen battery life. Moreover, during inactive periods, power to selected circuits may be cycled between on and off states in order to reduce power consumption. For example, power may be provided to digital signal processing (DSP) circuitry on the receiver circuit only when a signal is being received, such as during Time Division Multiple Access (TDMA) bursts. In this instance, between bursts and during transmit processes, the power supply to the DSP circuitry may be shut down (“collapsed”). By turning off the power to the DSP circuitry, overall power consumption is reduced, as compared to the power consumption of a DSP circuit that received a constant power supply. However, if the DSP circuit is used to perform the calibration operations and the power supply to the DSP circuit is turned off to preserve power, the error correction factor from the calibration operation may be lost.